1. Field of the Invention
The present invention relates to a variable valve gear for an internal combustion engine, which varies the open period of a valve.
2. Description of the Related Art
Regarding a reciprocating engine (internal combustion engine) installed in an automobile, a variable valve gear has been developed, which varies a valve-open-period according to the operational state of the engine in order to properly control the valve characteristics of intake and exhaust valves.
Many variable valve gears of this type have a configuration in which a cam lobe that is rotatably fitted onto the outer circumferential surface of a camshaft (cam drive shaft) supported by a cylinder head is combined with a valve-open-period variable mechanism that varies the rotational velocity of the camshaft at predetermined cycles to transmit the rotation to the cam lobe, as disclosed in Japanese Patent Gazette (Laid pen No. 10-280925). A lot of valve-open-period variable mechanisms have an Oldham coupling structure in which a drive arm is fixed onto the outer circumferential surface of the camshaft at a position adjacent to the cam lobe; an eccentric shaft is fitted to the outer circumferential surface of the camshaft at a position adjacent to the drive arm to be eccentrically rotatable; and a harmonic ring (intermediate rotary member) is rotatably fitted to the outer circumferential surface of the eccentric shaft. More specifically, the valve-open-period variable mechanism employs a structure that drives the valve by transmitting the constant velocity rotation of the camshaft, which is outputted from the drive arm, to the harmonic ring by using an input-side transmission member, changing the constant velocity rotation into non-constant velocity rotation that is changed in velocity at a predetermined cycle, and transmitting the rotation from a boss, which is protruding from the outer circumference of the end of the cam lobe, to the cam lobe by using an output-side transmission member. To phase-shift the axial position of the eccentric shaft from that of the camshaft adjusts a delay or advance of a rotational phase of the cam love relative to a rotational angle of the camshaft, and varies the valve-open period.
In order to place the valve-open-period variable mechanism in a limited area between each adjacent cylinders of the cylinder head, the mechanism has a structure in which the boss protruding from the outer circumferential surface of the end of the cam lobe is disposed close to a lateral portion of the drive arm in parallel with the lateral portion to carry out the transmission of the rotation from the harmonic ring, and the harmonic ring is supported by the eccentric shaft having a little larger external diameter than the camshaft located in the inside of an input gear portion of the variable mechanism.
As disclosed in the Japanese Patent Gazette, the valve-open-period variable mechanism is provided with a bearing portion such as a needle bearing in between the outer circumferential surface of the eccentric shaft and the inner circumferential surface of the harmonic ring for the purpose of smooth rotation of the harmonic ring.
The bearing portion can be displaced in an escaping direction, or more concretely, toward the cam lobe due to a change of the harmonic ring's movement (in result of a change in an eccentricity direction).
The displacement of the bearing portion disables the reliable support of the harmonic ring. Furthermore, the displacement causes abnormal abrasion. The bearing portion is disposed in a position eccentric to the camshaft to support the harmonic ring, and the boss of the cam lobe is located adjacent to the drive arm coaxially with the camshaft. Because of this displacement, the end face of the bearing portion repeatedly faces the end face of the drive arm and faces the end face of the boss of the cam lobe during the rotation of the harmonic ring. Especially the boss of the cam lobe is located outside the drive arm, so that the entire end face of the boss repeats a movement of deviating totally from the bearing portion to the outside and returning to the inside of the bearing portion. For this reason, if the end of the bearing portion, albeit slightly, projects from between the eccentric shaft and the harmonic ring, there generates such abnormal abrasion that the end of the bearing portion and the corner of the end face of the boss interfere with each other when the boss passes the end of the bearing portion.
The bearing portion can be prevented from escaping by employing a structure in which the bearing portion is fixed in between the harmonic ring and the eccentric shaft by press fitting or a structure in which a stopper is independently set in between the harmonic ring and the eccentric shaft.
In the case of the press fitting, however, it is difficult to reliably inhibit the axial movement of the bearing portion due to a high elastic deformability of the harmonic ring.
If the stopper is utilized, the bearing portion has to be made short in bearing length (strength decrease of the bearing portion) in order to secure a space for installing the stopper. In result, the sufficient support strength of the harmonic ring cannot be retained, which causes another problem.
A possible way of preventing the abnormal abrasion is to arrange the end face of the drive arm and that of the cam lobe to be flush with each other so that the boss smoothly passes between the end face of the bearing portion and that of the drive arm. Since the drive arm and the cam lobe are separate components and move differently, it is impossible to completely fit the end faces of these components together without a step or a gap. It is thus difficult to avoid the abnormal abrasion.